What different lines of evidence show that species have changed over time and share common ancestors?
Analyze and evaluate the evidence for evolution, including the fossil record, homologous and vestigial structures, and molecular (DNA and protein) similarities (TEKS Biology, Reporting Category 3; patterns; cause and effect).
A TEKS-level answer on the evidence for evolution for the Texas STAAR Biology EOC: the fossil record, homologous and vestigial structures, and molecular similarities, and how each line points to common ancestry and change over time.
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What this topic is asking
The Biology TEKS ask you to analyze and evaluate the evidence for evolution. For STAAR Reporting Category 3 you need to recognize the main lines of evidence (the fossil record, homologous and vestigial structures, and molecular similarities) and explain how each one points to common ancestry and change over time. This is a patterns and cause-and-effect topic, and it often appears as a question that gives you data or structures and asks what they suggest.
The fossil record
The fossil record shows that the kinds of organisms on Earth have changed over time: many species that once existed are now extinct, and forms found in older layers are often simpler or different from those in newer layers. Transitional fossils, which show features in between two groups, are especially strong evidence of gradual change. Because deeper layers are older, the order of fossils is a record of the sequence of life, a clear pattern over time.
Homologous and vestigial structures
Homologous structures are a powerful argument because there is no functional reason for unrelated species to share the same internal bone pattern; the simplest explanation is shared ancestry. Vestigial structures make the same point: a feature that no longer has its original use is best explained as a leftover from an ancestor where it did.
Molecular evidence
The most recent and precise evidence is molecular: comparing the DNA and protein sequences of different species. Because sequences are inherited and change slowly through mutation, species that share a more recent common ancestor have more similar sequences, while more distantly related species have accumulated more differences.
This is why molecular evidence so often agrees with the evidence from anatomy and fossils: all three reflect the same branching history of life. On STAAR, a table of sequence differences is a common stimulus, and the rule is simple: fewer differences means a closer relationship.
Putting the evidence together
No single fossil or structure proves evolution on its own. The strength of the case is that many independent lines of evidence agree: the fossil record, comparative anatomy, vestigial structures, and molecular data all point to the same conclusion, that today's species descended with modification from earlier ones. Evaluating evidence, weighing how well it supports a claim, is exactly the science practice this category tests.
Try this
Q1. Explain why homologous structures are evidence of a common ancestor. [2]
- Cue. They share the same underlying structure despite different functions; the simplest explanation is that they were inherited from a shared ancestor.
Q2. A table shows species A and B differ in DNA by 3 bases, and species A and C differ by 25 bases. Which pair is more closely related? [1]
- Cue. A and B, because fewer differences indicate a more recent common ancestor.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksThe forelimbs of a human, a whale, and a bat have a similar arrangement of bones but are used for different functions. These limbs are best described as which type of evidence for evolution? (A) Vestigial structures. (B) Homologous structures. (C) Fossils. (D) Mutations.Show worked answer →
A 1-point multiple-choice item on homologous structures.
The correct answer is B. Structures with a similar underlying arrangement but different functions are homologous structures, and they suggest a common ancestor. A vestigial structure (A) is a reduced, function-poor remnant, C is a different line of evidence, and D is the source of variation, not a structure.
Same basic structure, different use, points to common ancestry: that is homologous.
STAAR Biology (2024 SCR style)2 marksTwo species share a very similar DNA sequence for the same gene, while a third species has a very different sequence for that gene. Explain what this molecular evidence suggests about how closely related the species are. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response on molecular evidence.
Full credit (2 points): the two species with very similar DNA sequences are likely more closely related (shared a common ancestor more recently), because closely related species inherit similar sequences; the third species with a very different sequence is more distantly related, since more differences accumulate over more time since the common ancestor.
Partial credit (1 point): links similar DNA to closer relationship without explaining why differences accumulate over time. The science is scored.
Related dot points
- Explain how natural selection acts on heritable variation to produce adaptation in populations over time, and identify the conditions required for it to occur (TEKS Biology, Reporting Category 3; cause and effect; stability and change).
A TEKS-level answer on natural selection for the Texas STAAR Biology EOC: variation, overproduction, the struggle to survive, differential survival and reproduction, and how this leads to adaptation and change in populations over time.
- Recognize the factors that influence the genetic makeup of populations and lead to speciation, including mutation, gene flow, genetic drift, and reproductive isolation (TEKS Biology, Reporting Category 3; cause and effect; patterns).
A TEKS-level answer on the mechanisms of genetic change for the Texas STAAR Biology EOC: mutation, gene flow, and genetic drift as sources of change in a population, and how reproductive isolation leads to speciation.
- Describe how organisms are classified using a hierarchical taxonomic system based on shared characteristics, and use the levels from domain to species (TEKS Biology, Reporting Category 3; patterns; systems and system models).
A TEKS-level answer on classification for the Texas STAAR Biology EOC: the hierarchical taxonomic levels from domain to species, the three domains, binomial nomenclature, and how shared characteristics group organisms.
- Interpret cladograms and phylogenetic trees to determine evolutionary relationships based on shared derived characteristics and molecular evidence (TEKS Biology, Reporting Category 3; patterns; systems and system models).
A TEKS-level answer on cladograms for the Texas STAAR Biology EOC: how to read a cladogram or phylogenetic tree, what nodes and branches represent, how shared derived traits group organisms, and how to judge relatedness.
- Recognize the types of gene mutations and explain how a change in the DNA base sequence may be harmful, beneficial, or neutral and how it can be inherited (TEKS Biology, Reporting Category 2; cause and effect; stability and change).
A TEKS-level answer on mutations for the Texas STAAR Biology EOC: what a mutation is, substitution, insertion, and deletion, why an effect can be harmful, beneficial, or neutral, and how mutations in gametes are inherited and supply variation.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)